Plastic fasteners of the general type to which this invention relates have been available in a variety of forms, each particularly intended to satisfy a set of requirements peculiar to one type of application and to be suitable for some other applications; but every such fastener has had limitations along with its desirable features.
U.S. Pat. No. 4,083,289, to Erickson, issued in 1978, disclosed a plastic fastener of the general type here under consideration, comprising a plunger insertable into a sleeve-like body with forwardly projecting legs. The legs and the plunger had opposing wedging surfaces whereby the plunger held the legs diverged when it was in a forward operative position in the body. Although satisfactory for many purposes, the fastener of the Erickson patent was not entirely suitable for situations where severe vibration could be encountered, as in automotive applications. The wedging surfaces on the plunger and the legs cooperated to exert a rearward force upon the plunger that was normally resisted by friction between the plunger and the body member, but such friction was relieved by vibration, allowing the plunger to back out of the body.
U.S. Pat. No. 3,385,158, issued to L. H. Morin in 1968, disclosed another plastic fastener that was of the general type here under consideration. As with the fastener of the above discussed Erickson patent, the Morin fastener had a sleeve-like body with forwardly projecting legs and had cooperating wedging surfaces on the legs and on its cylindrical plunger whereby the legs were diverged by forward movement of the plunger to a fully inserted operative position. Small spur-like radially projecting lugs on the plunger were intended to engage against forwardly facing abutments in the body member when the plunger was in its operative position, to hold the plunger against backing out of the body member under the influence of vibration. The abutments on the body member were located in the slots between the legs, and therefore the plunger had to be inserted in a position of its rotation that maintained alignment of its lugs with those slots. Furthermore, the lugs had to be small enough to be compressively deformed as the plunger moved forward in the body member, since its bore could not be relieved to accommodate the lugs; and therefore the lugs could not be relied upon to make secure engagement with the abutments in the body member. Being so small, the lugs could easily be broken away or permanently deformed by repeated insertions and removals of the plunger, and therefore the fastener was not satisfactorily re-useable.
In most of the prior fasteners of the type here under consideration, the plunger and the body member were maintained in preliminarily assembled coaxial relationship by virtue of their being molded integrally with one another and connected by a thin flashing. The flashing was intended to be broken by the initial application of forward driving force to the plunger; but to ensure that the flashing would be thin enough to fracture cleanly, the plunger could initially project into the body member to only a very slight depth, and therefore the plunger received practically no guidance from the body member during the first part of its forward movement. As a result, a special tool had to be used, or special skill had to be exercised, to avoid tilting the plunger as it was driven forward. If a fastener of this type was reuseable, its plunger tended to become separated from the body member when the fastener was removed from a workpiece, so that re-installation involved the inconvenience of finding and manipulating each of the two parts of the fastener.
A somewhat improved form of reuseable fastener was disclosed in the above identified copending application, Ser. No. 224,955. Its sleeve-like body had a substantially tubular rear portion from which legs projected forwardly and inside which there were short circumferentially extending lands of semi-circular cross-section. The plunger had two circumferential grooves, one near its front end and one near its rear end, each having a semi-circular cross-section corresponding to that of the lands. The lands engaged in the front groove when the plunger was in its rearward projecting position and engaged in the rear groove when the plunger was in its forward operative position, thus cooperating with the grooves to provide detents that defined the two plunger positions. However, the lands had to be compressively deformed as the portion of the plunger between its grooves passed in engagement with them, and such compression was not suitable where the fastener had to be frequently withdrawn and reinstalled. Furthermore, if the plunger was for any reason not fully inserted into the body, so that the lands were only partially seated in the rear plunger groove and remained under compressive deforming stress, the lands could eventually take a set that allowed the plunger to creep rearward in the body under the influence of vibration.
The sleeve-like body of a fastener of the type here under consideration usually has a radially enlarged head at its rear end that defines the limit of its forward insertion into a hole in a workpiece. Usually there is also a head on the rear end of the plunger. In prior fasteners, when the plunger was in its forward operative position in the body, the head on the plunger would overlie or partly overlie the body head in such a manner as to present the appearance of a knob-like bulge or protuberance on the body head, and there was usually a circumferential crack or crevice between the plunger head and the body head in which dirt could settle. With fasteners that were in hidden or inconspicuous locations, this unsightly appearance of the heads was of no consequence, but fasteners of the type here under consideration are coming into increasing use for securement of automobile body interior door panels, roof liners and the like, in installations where the fastener heads are fully visible. Heretofore fasteners intended for such applications have been designed solely with regard to functional and utilitarian considerations, although it was obvious that they were unsatisfactory from an aesthetic standpoint. Apparently it was not obvious how to arrange such a fastener so that the visible part of it would be attractive while still meeting all of the functional requirements, including the requirement that the plunger be arranged for ready retraction in the sleeve-like body in order to permit the fastener to be removed from a workpiece and reused.